Seismic stratigraphy and tectonics of Late Miocene basin fill in southern Transdanubia and below Lake Balaton

Major progress in understanding the stratigraphy of the postrift (Late Miocene to Quaternary) strata in the Pannonian Basin has been achieved in the past two decades, but a few basic questions remain unsolved. These are associated with the controversial presence of third-order water-level oscillations in Lake Pannon and large-scale tectonic deformation and erosion of the basin fill. Sequential stratigraphic interpretation was carried out in southern Transdanubia using hydrocarbon exploration seismic sections and borehole data. The Late Miocene basin fill was divided into 5 third-order depositional sequences as follow: SAR-1, PAN-1, -2, -3, -4. In the Drava trough, the topmost sequence boundary is overlain by more than 1000 mthick Pliocene through Quaternary strata. These were deposited in on alluvial plain and are characterised by higher order cyclicity. A special contribution to our seismic stratigraphic interpretation was offered by a wealth of high-resolution seismic profiles taken at Lake Balaton over the last two decades. These surveys imaged the Pannonian Szák, Somló and Tihany Formations to thicknesses ranging between 0 to 120 m below the mud beds of the lake and above the acoustic basement given by the top of Sarmatian limestone layer. According to the sequential stratigraphic interpretation, these formations were deposited on the shelf of Lake Pannon during the PAN-2 sequence and they represent transgressive, highstand and falling stage system tracts. Regression of Lake Pannon led to the formation of a large alluvial/delta plain as evidenced by the upper section of the Tihany Formation. These terrestrial beds in the Tihany Peninsula are overlain by pyroclasts, maar lake sediments (Tihany Volcano) and freshwater limestones; the latter have been silicified locally due to postvolcanic, hot spring activity. All of these exposed features can be recognised on the high resolution seismic sections of Lake Balaton. In addition, seismic data demonstrate that the top of the Tihany Formation is a marked erosional unconformity. Accordingly, it is reasonable to conclude that this seismic unconformity represents the upper boundary of the PAN-2 sequence. Seismic sections in Transdanubia show that the Late Miocene depositional sequences suffered remarkable postsedimentary compressional deformation. This Pliocene through Quaternary compression can be considered as the neotectonic phase of the evolution of the Pannonian Basin. It has resulted in development of areas of uplift and coeval subsidence with an amplitude of the order of 1000 metres. Uplift and erosion of the Transdanubian Range also took place during this phase. Elsewhere in the basin eroded material from the uplifting terraines filled up the areas of subsidence and no significant topographic relief has been formed. It is reasonable to infer that the start of the structural inversion, erosion and fault reactivation in the Pannonian Basin at around the Miocene-Pliocene boundary is a tectonic Messinian event. Finally, it was concluded that the the longstanding problem of the correlation of the basin margin and deep basin Pannonian lithostratigraphic formations remains one of an intractable nature unless the large-scale deformation and erosion of the basin-fill during the neotectonic inversion are taken into consideration more thoroughly.